Ga—N-based light emitting diodes (LEDs) have been intensively studied and developed in recent years. High efficiency, high power, GaN-based LEDs have attracted interest for applications such as displays, traffic signals, back lighting for mobile/cellular telephones and similar apparatus, and white light sources. Reducing cost and improving light output efficiency are important factors to enable such GaN LEDs to succeed in the mainstream lighting market.
In general, the internal quantum efficiency (ηi) for GaN-based LEDs is significantly less than 100% due to crystal quality and epitaxial layer structure. A typical (ηi) can reach about 70 to 80%. Further improvement has proven difficult to achieve. The external quantum efficiency (ηext) is still much lower than internal quantum efficiency. This is because the light extraction efficiency of conventional GaN-based LEDs is limited by total internal light reflection, which occurs at the semiconductor-air interface due to the high refractive index of GaN (n≈12.5) compared to air (n=1). The critical angle for the light generated in the active region is only about 23°. Most of the light generated is repeatedly reflected into the substrate and eventually absorbed. Assuming that light emitted from sidewalls and the bottom is neglected, only a small fraction (4%) can be extracted from the surface.
Conventional GaN-based LEDs grown by metalorganic chemical vapor deposition (MOCVD) use a nonconductive sapphire substrate. The epitaxial layers on the sapphire substrate consists of usually a light-generating layer (active region) sandwiched between a relatively thick n-type doped GaN layer and relatively thin p-type doped GaN layer. The n-type GaN layer is formed by a stack of multiple layers (undoped or doped to n-type semiconductor made of GaN related materials like GaN, AlGaN, or InGaN, or AlGaInN, etc.) on the sapphire, while the p-type GaN layer is formed by a stack of multiple layers (undoped or doped to p-type semiconductor made of GaN related materials like GaN, AlGaN, or InGaN, or AlGaInN, etc.) away from the sapphire. The top p-GaN surface epitaxial layer is Ga-Polar which is often used as light extraction surface. The poor thermal conductivity of the sapphire substrate, and the relatively high current densities, combine to degrade the device performance due to excessive heating from the active layer during operation. At the same time, the relatively thin p-GaN layer (usually less than 0.5 micrometer) and the high resisitivity of p-GaN, is highly sensitive to plasma damaging and is difficult to use for dry surface texturing. Furthermore, Ga-polar GaN is chemically inert and is more difficult to wet etch than N-polar GaN. The other side of the active region, i.e., the n-GaN layer of the active region is usually much thicker (2 to 5 micrometers thick) than the p-type GaN layer, and is ideal for making surface texturing due to its thickness. However, this part is below the active region and on the sapphire. It is not able to be surface textured unless the sapphire is removed.
To address these problems, vertical laser liftoff of GaN LEDs and other methods have been developed to detach the sapphire from the GaN epitaxial films grown on it. Flip-chip or other bonding technologies have also been developed to attach the GaN films to a new substrate with good thermal conductivity. Different surface roughening techniques on exposed LED N-polar n-GaN surface have also been developed, including ICP plasma etching and wet etching.
The formation of micro-lenses on an output surface of a light emitting diode has been proposed. However, in the main it is not possible as the active region is close to the light emitting surface on the p-type GaN layer and the forming of the micro-lenses or surface roughing may damage the active region.